The use of optical interferometry to measure changes in either length, distance, or optical length has grown significantly due not only to technological advances in lasers, photosensors, and microelectronics but also to an ever increasing demand for high precision, high accuracy measurements. The prior art interferometers can be generally categorized into two types based on the signal processing technique used, i.e., either homodyne or heterodyne, see for example, H. Matsumoto, "Recent Interferometric Measurements Using Stabilized Lasers", Precision Engineering, Vol. 6, pp. 87-94 (April 1984). The interferometers based on the heterodyne technique are generally preferred because (1) they are insensitive to low frequency drift and noise and (2) they can more readily have their resolution extended. Within the heterodyne type of interferometers, of particular interest are the ones based on the use of two optical frequencies, see for example, G. Bouwhuis, "Interferometrie Met Gaslasers", Ned. T. Natuurk, Vol. 34, pp. 225-232 (August 1968); Bagley et al, U.S. Pat. No. 3,458,259, issued July 29, 1969; Hewlett Packard Journal (August 1970); Bagley et al, U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; Hewlett Packard Journal (April 1983).